The invention relates to plasters, i.e., materials which are to be applied to walls of rooms within buildings for practical and/or decorative purposes. The present invention relates more particularly to plasters for use on walls in which there exists a relatively high himidity and is thus especially relevant for use in the reclaiming and renovating of old buildings. The masonry to which the plaster is applied may contain a limited amount of inherent humidity but the masonry may also be subject to a continuous exposure to humidity such as, for example, bath houses, breweries etc. Similarly, the building may contain masonry that is in contact with moisture that is present in or near the foundation, the moisture being caused to rise into the masonry due to capillary forces. The latter condition is very common in old buildings.
If ordinary plaster is applied to such masonry, the outer surface is generally sealed with respect to the internal humidity but the humidity is retained in the wall end continues to act therein. Accordingly, it has been attempted for a long time to discover various plasters which permit the water contained in the masonry to travel to the exterior so that it may evaporate there. A critical property of a plaster which is to perform this function is its coefficient of diffusion resistance, i.e., the resistance which the plaster offers to the diffusion of water from the masonry through the plaster to its surface where it may evaporate into the atmosphere. The plaster must also and at the same time exhibit satisfactory insulation to heat and sound however.
Commonly known lime plasters have a coefficient of diffusion resistance of approximately 15-18 and cement mortar plasters have a coefficient of approximately 22-25. Accordingly, lime plasters would be more suitable in principle to perform the diffusion of moisture because they have a lower inherent resistance to its passage than do cement mortars. However, they have the unfortunate property of releasing the moisture only slowly and thus permitting only slow drying rates. Furthermore, lime plasters are susceptible to the growth of fungus. On the other hand, cement mortars are very resistant to fungus growth but their other properties make them unsuited for dehumidifying functions.
It has been attempted to provide plasters with an increased degree of capillary activity, i.e., plasters with a relatively loose structure which would permit a greater permeability to moisture and such plasters have exhibited coefficients of diffusion resistance of approximately 11 for a normal thickness of approximately 15 mm. However, even this improved diffusion resistance does not suffice to dry out masonry walls, especially if water is continuously resupplied by capillary forces from the foundation of the building.